Machine tools, more particularly lathes, are subject to thermal expansion of the machine system, causing the datum or zero positions of the spindle (workpiece), a tool measuring device and a tool holder, to drift relative to each other. In combination the various drift movements result in improper tool positions relative to the workpiece. This produces positional errors in an amount up to and occasionally exceeding 100 .mu.m.
This requires the machine operator to readjust the tool position repeatedly. Additionally measurements and test cuts must be made, and if necessary, data from prior experience must be entered in the calculation of the readjusted position.
The master elements conventionally used for calibration must be mounted to run with the requisite degree of accuracy on the spindle of the machine tool which requires a comparatively great setup effort (changing the setup and positioning) plus time-consuming calibration work, all of which are associated sources of error.
DE-PS 35 30 567 discloses a method and device for mechanically sensing the tool position in which a reference column is attached directly to the lathe to eliminate the need for mounting a reference ring in the chuck, the reference column being resiliently arranged on the machine in a calibrated position relative to the workpiece. In this arrangement, ultrasonic vibrations can be produced in the reference column, and an accelerometer arranged on the turret of the respective lathe responds to the vibrations when a tool acting as a mechanical sensor comes into contact with the reference column. The accelerometer ouput signal is then transmitted from the turret by a rotary coupling, the signal is treated and then is applied to a numerical control unit for use either in calibration or in the measurement of parts being machined.
The above arrangement seeks to provide improvements in the measuring of test cuts by using the respective tool and no other means to measure the test cuts in all positions. In the measurement of the workpiece, however, there is assumption of an actually existant firm and rigid zero position, so that corrections of the positions of the tool tip are influenced by error induced by thermal drift, so that this arrangement cannot obviate the need for test cuts.